The global market of fixed false tooth grows continuously. In terms of the fabrication of fixed false tooth, the patient's tooth must be prepared before gingival retraction to expose the ground tooth hidden in the gum before impression. However in the impression process, the accuracy, dimensional stability and operating variable can influence the precision of the finished false tooth directly.
The dental impression methods include traditional impression and digital impression. In terms of traditional impression, the first step is gingival retraction to expose the ground tooth hidden in the gum, so as to avoid impressing the tooth and gum at the same time that leads to unprepared margin impression of tooth, and then the false tooth margin made according to this impression will be imprecise. The gingival retraction modes include mechanical and mechanical-chemical modes. The former one simply pushes away the gum laterally with mechanical force. The latter one uses special cotton thread and chemical gingival retraction solution. There are two main classes of chemical gingival retraction solution, which are vasoconstrictor and astringent, both of them are effective on hemostasis, so they are favorable for impression taking. The dentists mostly use the gingival retraction method of mechanical-chemical gingival retraction cords for gingival retraction. The gingival cord is pressed in the gingival trough, the gum is pushed away horizontally with mechanical force, and the exudate and blood from the gingival trough are controlled by the chemical gingival retraction solution in the cord. This method shall press the gingival cord in the fine gingival trough with extruding force, so as to separate the tooth from the gum. The gingival cord is extracted during impression, thus the dental margin on the impression is very clear, and the dental impression technician can make the false tooth accurately according to this model. In the course of gingival retraction, the special gingival cotton thread for arresting bleeding shall be squeezed into the gingival trough, so the gingival retraction process not only takes time, but also discomforts the patient, and there are problems, such as unclear tooth shaping and gingival retraction in the treatment area, bleeding and intraoral moisture control in the treatment area, dimensional fit of impression tray, operation localization of impression tray, the blocking and discomfort of the patient's mouth resulted from the impression tray, insufficient impression material operating time, quality and contamination of impression material, mixing ratio of impression material, patient's nausea resulted from peculiar smell of impression material, deformation resulted from different flow paths and mixing ratios of gypsum, and the errors resulted from the manual finishing of gypsum pattern.
The most popular digital impression technique has been developed for three-dimensional appearance of tooth, which is more sanitary, convenient and faster than traditional impression technique. At present, many manufacturers have developed the digital impression technique. There are two main forms, one uses blue Light Emitting Diode (LED), this method needs a contrast medium or powder to obtain the tooth surface configuration. The other method uses laser technology for scanning, and the tooth surface is measured to acquire image. However, said two digital impression techniques cannot obtain the contour information of the tooth under the gum before gingival retraction. Therefore, to make the false tooth by using current digital impression technique for the patient, said gingival retraction method of gingival retraction cord shall be used for gingival retraction, so as to obtain the tooth contour hidden in the gum. In other words, the traditional impression or current digital impression requires said gingival retraction method of gingival retraction cord for gingival retraction, thus, the patient is likely to have gingival bleeding, and the patient is discomforted in the fabrication of false tooth, and the patient's periodontal ligament may tear.